The long-term objective of this proposal is to structurally define type II topoisomerase function and drug inhibition mechanisms. Type II topoisomerases are a ubiquitous class of proteins that use ATP to actively transport one DNA duplex through another. This reaction maintains appropriate levels of DNA supercoiling and resolves lethal chromosome tangles in the cell. Eukaryotic type II topoisomerases are also exploited by certain inhibitors that are frontline clinical therapies for cancer. The focus of this proposal is on eukaryotic topoisomerase II and archaeal topoisomerase VI, two representative members of type IIA and liB topoisomerases, respectively. The specific aims of the research are as follows: 1. To understand how type II topoisomerases physically switch between conformational states. 2. To define how type II topoisomerases interact with and utilize reaction substrates. 3. To determine how certain anticancer agents bind to and inhibit eukaryotic topoisomerase II Although a general framework exists for understanding type II topoisomerase function, significant gaps remain. A detailed structural description of type II topoisomerase mechanism coupled with biochemical validation, will illuminate critical aspects of these enzymes' function. The studies outlined here will define the physical events by which type II topoisomerases catalyze the ATP-dependent passage of one DNA segment through another and by which anticancer inhibitors subvert enzyme function. Data resulting from such efforts broadly impact a number of important scientific research fronts, from understanding how a molecular machine maintains chromosome topology to dissecting the molecular basis of various chemotherapeutic treatments.